Chemists at the University of California Berkeley have developed a way to store methane that they claim could speed the development of natural gas-powered cars that do not require the high pressures or cold temperatures of the current compressed (CNG) or liquefied natural gas (LNG) vehicles.

Cross-section through a flexible MOF shows how the chemical structure shifts when methane is absorbed. Image credit: Jarad Mason.To date , those two factors have combined to limit the amount of, and ease with which methane fuel can be accommodated in passenger cars. This also has limited market penetration of cleaner-burning natural gas into that segment of the vehicle market.

Chemists led by UC Berkeley professor Jeffrey Long have developed a porous and flexible material—a metal-organic framework (MOF)—for storing methane that addresses these problems. The flexible MOF collapses when the methane is extracted to run the engine, but expands when the methane is pumped in at moderate pressure, typically within the range produced by a home compressor.

“You could potentially fill up at home,” says Long.

The flexible MOF can be loaded with methane, the main ingredient of natural gas, at 35 to 65 times atmospheric pressure (500-900 psi). By contrast, CNG vehicles compress natural gas into an empty tank at 250 atmospheres (3,600 psi).

LNG vehicles operate at lower pressures, but require insulation in the tank system to maintain the natural gas at -162 degrees Celsius (-260 degrees Fahrenheit) so that it remains liquid.

Long says that next-generation natural gas vehicles will require a material that binds the methane and packs it more densely into the fuel tank, providing a greater driving range. One problem has been finding a material that absorbs the methane at a relatively low pressure, such as 35 atmospheres, but gives it all up at a pressure where the engine can operate, at between 5 and 6 atmospheres.

MOFs, which have a lot of internal surface area to adsorb gases—that is, for gas molecules to stick to the internal surfaces of the pores—and store them at high density, are a promising material for adsorbed natural gas storage.

Among the other advantages of flexible MOFs, according to Long, is that they do not heat up as much as other methane absorbers, so there is less fuel cooling required.

“If you fill a tank that has adsorbent, such as activated charcoal, when the methane binds it releases heat,” he says. “With our material, some of that heat goes into changing the structure of the material, so you have less heat to dissipate, less heat to manage. You don’t have to have as much cooling technology associated with filling your tank.”

The flexible MOF material could potentially be placed inside a balloon-like bag that stretches to accommodate the expanding MOF as methane is pumped in, so that some of the heat given off goes into stretching the bag.

Long has been exploring MOFs as gas adsorbers for a decade, hoping to use them to capture carbon dioxide emitted from power plants or store hydrogen in hydrogen-fueled vehicles, or to catalyze gas reactions for industry. However, last year a study by UC Berkeley’s Berend Smit found that rigid MOFs have a limited capacity to store methane. Long and graduate student and first author Jarad Mason turned to flexible MOFs, noting that they behave better when methane is pumped in and out.

The flexible MOFs tested are based on cobalt and iron atoms dispersed throughout the structure, with links of benzenedipyrazolate (bdp). Both cobalt bdp and iron bdp)are highly porous when expanded, but shrink to essentially no pores when collapsed.